4 August 2009

Converting hydrogen to metal

The most common element in the universe, hydrogen, is normally an insulating gas, but at high pressures, it may turn into a superconductor.

A hydrogen-based compound now in development could help in the search for metallic and superconducting forms of hydrogen.

The hydrogen based compound SiH4(H2)2 may be a useful system in which to explore metallic hydrogen.

Hydrogen is the simplest of the elements: It contains one proton and one neutron. Because hydrogen is so light, quantum theory said it will have a significant energy even when it cools to very low temperatures. This is why hydrogen only solidifies at 14 degrees above absolute zero.

Scientists at the Carnegie Institution in Washington D.C., predicted it should be possible to form a metal from hydrogen, but the pressure needed to do so—some 4 million atmospheres—exceeds that at the center of the earth. However, by forming compounds of hydrogen with another element like Si, it is possible to make fairly dense forms of hydrogen that do become metals at more experimentally accessible pressures. In fact, SiH₄ becomes a metal at one tenth the pressure needed to make pure hydrogen metallic, and a superconductor at 1 million atmospheres.

Carnegie scientists Timothy Strobel, Maddury Somayazulu, and Russell Hemley conducted extensive high-pressure experiments on a mixture of SiH₄ and H₂. At pressures of only ~ 7.5 GPa, they discovered a new compound—SiH₄(H₂)2—in which the hydrogen bonds are unusually weak and which may become a metal at higher pressures.

The ultimate goal is to generate conditions under which hydrogen effectively becomes metallic, and hopefully superconducting, at pressures lower than those required for pure solid hydrogen.

For related information, go to www.isa.org/manufacturing_automation.